1. Field of the Invention
The present invention relates to a duplicating apparatus, and more particularly a duplicating apparatus for forming a plurality of copies having the same image by repeatedly developing one and the same electrostatic latent image.
2. Related Art Statement
Heretofore, there has been practised a duplicating apparatus in which after an electrostatic latent image has been once formed on a charge retentive member, development and transfer are repeatedly effected to form a plurality of copies from the same latent image. In such a retention type duplicating apparatus it is important that the electrostatic latent image should not be deteriorated or decayed during the repetitive development and transfer. For instance, when the latent image is developed by a magnetic brush developing device using a two component developing agent consisting of magnetic carriers and toners, it has been tried to avoid a leakage of electrostatic charge by increasing the resistance of the developing agent or adjusting the developing bias voltage, and to avoid fog which is induced on an imagewise bright area by an injection of charge due to friction between the developing agent and a charge retentive member, e.g. a photosensitive body. However, such known measures could not provide a satisfactory solution, and usually it is quite difficult to obtain more than thirty duplicated copies from the single latent image.
In order to mitigate the above mentioned drawbacks, in Japanese Patent Kokai Sho No. 56-119,142 there has been proposed a non-contact type developing device comprising a developing roller having an AC bias applied thereto. In such a developing device, toner particles are alternately moved toward the charge retentive member and toward the developing roller by means of a transporting electric field and an anti-transporting electric field, which are generated alternately by the AC bias applied to the developing roller. In this device, it is possible to obtain a duplicated copy having a relatively good image quality. However, in this known device, since the AC bias voltage is applied directly to the developing roller, the latent image potential is reduced to about two-thirds of the original surface potential after forming about forty copies from the same latent image.
In other electrophotographic apparatuses than the above-mentioned retention type apparatus, the reproducibility of a fine line is not satisfactory and a density of the fine line is not high. The density of the fine line can be increased by utilizing the edge effect. However, if the edge effect is used, there occurs another problem in that a large black area could not be reproduced at a constant density. Further, in the known duplicating apparatus, a so-called .gamma.-characteristic of the density of the reproduced image is too steep to obtain a good gradation.